Optical data storage system have attractions as an alternative to those based on magnetic tapes and discs, principally because of their higher storage density, which in theory is only limited by the optical diffraction limit of light waves, and their potentially higher data input rates.
Holographic recording techniques have been proposed for archival storage of data using silver halide photographic films as the recording media but such systems are inherently non-reversible so that the recorded information cannot be altered and is therefore not suitable for the great bulk of data storage applications. Also even in the case of very high resolution photographic films, the grain size of the silver halide particle placed a serious limited on the storage capacity of the film.
While photochromic compounds might be regarded as possible substitutes for a silver halide emulsion, as a data recording media, such compounds are generally found to be unsuitable for a variety of reasons. For example, known photochromic compounds generally exhibit poor thermal stability and undergo non-reversible side reactions on photoactivation so that their sensitivity quickly diminishes over a few cycles.
The present invention utilises the unusual properties of a series of photochromic succinic anhydride and succinimide derivatives having the following general formula: ##STR1## wherein x represents oxygen or &gt;NR.sub.6, R.sub.6 being hydrogen or an alkyl, aryl or aralkyl group;
R represents an alkyl, aryl, aralkyl or heterocyclic group; PA1 A represents a 3-furyl, 3-thienyl, 3-benzofuryl or 3-benzothienyl group; PA1 B represents an adamantylidene group or the grouping ##STR2## in which R.sub.2 and R.sub.3 independently represent an alkyl, aryl, aralkyl or heterocyclic group or one of R.sub.2 and R.sub.3 represents hydrogen and the other represents an alkyl, aryl, aralkyl or heterocyclic group. PA1 R represents an alkyl, aryl, aralkyl or heterocyclic group; PA1 A represents a 3-furyl, 3-thienyl, 3-benzofuryl or 3-benzothienyl group; PA1 B represents an adamantylidene group or the grouping ##STR5## in which R.sub.2 and R.sub.3 independently group or one of R.sub.2 and R.sub.3 represents hydrogen and the other represents an alkyl, aryl, aralkyl or heterocyclic group. PA1 R represents hydrogen or an alkyl, aryl, aralkyl or heterocyclic group; PA1 Ar represents a 3-furyl, 3-thienyl group, 3-benzofuryl or 3-benzothienyl group, or a phenyl group containing a meta alkoxy or aryloxy substituent; and PA1 Ad represents an adamantylidene group. PA1 R represents hydrogen or an alkyl, aryl, aralkyl or heterocyclic group; PA1 Ar represents a 3-furyl, 3-thienyl group, 3-benzofuryl or 3-benzothienyl group, or a phenyl group containing a meta alkoxy or aryloxy substituent; and PA1 Ad represents an adamantylidene group.
Compounds of the general formula (1) above undergo photocyclisation when exposed to light in the ultra-violet range, ring closure occurring between the carbon atom to which the groups R.sub.2 and R.sub.3 are attached and the 2-position of the furyl or thienyl ring. The photocyclisation reaction is illustrated by the following typical case: ##STR3## Under the influence of U.V. light the compound (X) is converted in high yield to the cyclic form which is stabilised as shown in forms (XA) and (XB).
The cyclic forms of the compounds of general formula (1) are deeply coloured, usually exhibiting a visual effect in the bright red to deep purple range, and this is believed to arise from the double bond structure extending from the oxygen or sulphur heteroatom at one end to the succinic anhydride carbonyl oxygen at the other.
In addition to possessing good thermal stability and low fatigue (relative freedom from irreversible photochemical side reactions) the compounds of formula (I) exhibit a high rate of conversion to the coloured form which is deeply coloured. The last mentioned properties are very important in an optical recording medium since it must be recognised that there is no "gain" when a photochromic compound is exposed to light of the appropriate wavelength. One activating photon can only convert one molecule of photochromic compound to its coloured form whereas one photon will in photographic emulsion cause an entire grain of silver halide to be reduced to silver after development. Thus, it will be appreciated that the high sensitivity and deeply coloured forms of the photochromic compounds are of considerable significance in their use as optical recording media.
The compounds of Formula (I) above are disclosed in U.S. patent application Ser. No. 846,148 (Harold G. Heller), and the disclosure of that application is incorporated herein by reference.
According to one aspect of the present invention there is provided a holographic data recording and storage method which comprises subjecting a photochromic recording material to radiation whereby the material is converted to its coloured form and forming a holographic record using coherent visible light by interfering a reference beam and a data beam in the plane of the surface of the material whereby a series of holograms embodying data recorded from said data beam are formed by bleaching said material, said photochromic material comprising a substrate having coated thereon or dispersed therein a photochromic compound of the general formula: ##STR4## wherein x represents oxygen or &gt;NR.sub.6, R.sub.6 being hydrogen or an alkyl, aryl, or aralkyl group;
U.S. patent application Ser. No. 846,148 describes a sub-class of compounds falling within the above general formula (I) above in which B represents an adamantylidene group. This sub-class of compounds possess a structure which is very stable and free from both angle and conformational strain and in which bond migration does not occur. It has been found that compounds of this sub-class are particularly useful for holographic recording purposes.
According to a further aspect of the present invention there is provided a holographic data recording and storage method which comprises producing a holographic record using coherent visible light by interfering a reference beam and a data beam in the plane of the surface of the material, whereby a series of holograms embodying data recorded from said data beam are formed, said photochromic material comprising a substrate having coated thereon or dispersed therein a photochromic compound of the general formula: ##STR6## wherein X represents oxygen or &gt;NR.sub.6, R.sub.6 being hydrogen or an alkyl, aryl, or aralkyl group;
Preferably R in formula (I) and (II) above represents a lower alkyl (e.g. having 1 to 6 carbon atoms), a phenyl, napthyl or alkylphenyl group.
Formation of the hologram by bleaching (i.e. by conversion of the photochromic compound from its cyclic to non-cyclic form) is of great practical significance since it enables the powerful emission lines of the argon ion laser at 488 or 514 mm to be used for writing the data. The term bleaching is used to describe this conversion, although in some cases the photochromic compound may have a pale colour in its non-cyclic form.
Conversion of the compound to its coloured state may be achieved by off line exposure to a U.V., e.g. mercury vapour lamp or, in the case where an argon ion laser is used, by operating the laser simultaneously in the visible and U.V. ranges and directing the U.V. beam onto the photochromic surface so as to activate the surface prior to writing the data by bleaching.
The invention includes a holographic data recording and storage method which comprises a holographic record using coherent visible light by interfering a reference beam and a data beam in the plane of the surface of the material whereby a series of holograms embodying data recorded from said data beam are formed, said photochromic material comprising a substrate having coated thereon or dispersed therein a photochromic compound of the general formula: ##STR7## wherein X represents oxygen or &gt;NR.sub.6, R.sub.6 being hydrogen or an alkyl, aryl, or aralkyl group;